The present invention concerns valves of the type having a rotatable valve stem passing through a valve body and having a sealing surface for engagement and disengagement with the valve seat. Such valves are commonly used in many industries, and one type of valve referred to as a control valve is illustrated in several embodiments in the brochure entitled "WKM CATALOG WHS-78" distributed by WKM Wellhead Systems, Inc. Many such valves are provided with a bonnet fixed to the valve body and adapted for receiving the rotatable valve stem, and for convenience in description are the type of valve generally referred to below.
More particularly, the present invention concerns the seal conventionally provided between the valve bonnet and rotatable valve stem, which seal prevents the escape of pressurized fluid from the valve body. Many such seals have been heretofore devised, and probably the most commonly used seal is formed from packing material which is placed between the valve stem and the bonnet. Although packing materials of various types generally provide an adequate seal, it is commonly known that periodic maintenance is generally required to maintain such a seal. Such maintenance may consist of tightening the packing glands or replacing the packing material, and such maintenance is costly both in terms of time and equipment downtime.
Other types of seals between the bonnet and stem have been devised, such as a lip seal having a generally U-shaped cross-sectional configuration. Fluid pressure between the lips or uprights of the U-shaped seal force the outer lip outwardly into engagement with the bonnet, and force the inner lip inwardly into engagement with the stem. Advantages of such a lip seal are that little or no maintenance is required, and the sealing force between the seal and the bonnet and stem increases as the fluid pressure in the valve increases.
A major problem with the use of lip seals between the bonnet and stem, however, concerns the ability of the seal to provide the desired sealing function under relatively low pressure over a long period of time. Such seals typically are formed from a non-corrosive plastic, such as Teflon.RTM., which does not rapidly deteriorate when subjected to various fluids under a typical range of pressures and temperatures. When first installed, the lip seal material is sufficiently elastic to deformed under high fluid pressure for proper sealing engagement with the bonnet and stem, yet sufficiently resilient to return to its original configuration under relatively low pressure to maintain the desired sealing engagement. After being subjected to various pressure and temperature levels, however, satisfactory plastic lip seals tend to loose their resiliency, especially over time, and problems are encountered in maintaining the desired seal under low pressure.
One solution to the above problem with lip seals has been to provide a plastic lip seal with a metal leaf spring having a V-shaped cross-sectional configuration positioned between the inner and outer lips of the seal. Such a lip seal, referred to as a leaf spring energized seal, has been manufactured and sold by Polydyne Industries, Inc. in Denver, Colorado. Under relatively low fluid pressure, the leaf spring spaced between the inner and outer lips biases the outer lip into engagement with the bonnet, while the inner lip is biased into engagement with the stem. Under relatively high pressure, the effect of the leaf spring compared to the fluid pressure force is minimized, and the increased fluid pressure is a primary factor causing sealing engagement of the lips with the bonnet and stem.
The design of many valves is such, however, that there is very limited space between the bonnet and the stem, which tends to limit the use of leaf spring energized seals. Also, it should be understood that the manufacture and assembly of a leaf spring energized seal as described above, may be relatively costly, especially for low volume productions. The leaf spring energized seals described above have therefore not been commonly used on relatively small valves wherein little spacing is provided between the bonnet and the stem.
Further attempts have thus been made to provide an improved seal between the bonnet and the stem of the valve. In an attempt to retain the concept of the lip seal, various attempts have been made to devise a material for forming lip seals which has the desired sealing properties, including continued high resiliency with time over various pressure and temperature ranges. To date, however, such attempts have not been totally satisfactory. One such attempt has been to manufacture the lip seal from two different plastic materials: the first designed principally for ideal sealing purposes, and the second inner material to serve primarily as a continuous resilient material acting upon the outer material. This solution also has not been universally accepted, however, in part because the inner resilient material tends to deteriorate under adverse environmental conditions. The use of lip seals between the valve bonnet and valve stem has thus achieved limited success in the valve industry, and various other attempts have been made to provide the desired seal without utilizing the lip seal concept.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are herein provided employing the lip seal concept to seal the space between the valve body and stem.